The present invention relates to a method of producing shock-induced nanocomposite coatings, and more particularly to a method of producing shock-induced nanocomposite coatings with thermal or plasma spraying.
A xe2x80x9cnanocomposite coatingxe2x80x9d is a coating having more than one solid phase, in which at least one phase is in the nanometer range. Attention has been directed to nanoparticles and nanocomposites because of the unique properties exhibited by these materials. For example. Silicon (Si) is an indirect band gap semiconductor that can be potentially used for optoelectronic applications such as light emitting devices. Unfortunately, the development of such devices has been hindered since crystalline Si is not an efficient light emitter. However, this changed with the development of porous-Si (por-Si), an irregular network of nanocrystalline Si which exhibits a band gap energy twice the band gap energy of crystalline Si (L. T. Canham, Appl. Phys. Lett. 57(10): 1046-1048 (1990)). Thus, the potential applications of semiconductor materials, such as Si, have increased because of the development of nanomaterials.
Typically, nanocrystalline or nanocomposite coatings are produced using chemical techniques such as Chemical Vapor Deposition (CVD), which require gaseous reactants and vacuum chambers to contain the gaseous reactants. However, the use of these gaseous reactants involves safety risks, in addition to time and cost considerations associated with containment of the gaseous reactants. Thus, for mass scale production of nanocrystalline or nanocomposite coatings CVD techniques are not without disadvantages.
An common alternative to CVD is thermal or plasma spraying, which uses a particulate precursor material rather than a reactive gas. Thermal or plasma spray provides a flexible, cost-effective and safer method for producing coatings since gaseous reactants are avoided. Moreover, vacuum chambers are generally avoided since the coatings are typically sprayed at atmospheric pressure.
However, nanocrystalline or nanocomposite coatings produced solely with plasma or thermal spraying have yet to be developed. While combinations of CVD and thermal spraying have been used to produce nanocrystalline coatings (Heberlein et al., Thermal Spray: A United Forum for Scientific and Technological Advances, 329-333 (1997)), reactive precursor gases are still required to form the nanocrystalline coating.
In view of the current state of the art, there is a need for a method of producing nanocrystalline or nanocomposite coatings without reliance on gaseous precursor reactants.
Accordingly, it is an object of the present invention to provide a method of producing nanocomposite coatings without the use of reactive precursor gases. It is also an object of the present invention to provide a method of producing nanocomposite coatings having metastable, high pressure phases of nanocrystalline material.
The present invention provides a method of producing a nanocomposite coating which avoids the disadvantages associated with gaseous precursor reactants. The method includes providing a thermal or plasma spray apparatus capable of generating a high-velocity gas jet, providing a substrate to be impinged by the gas jet, generating the high-velocity gas jet and introducing into the gas jet a particulate containing a polymorphic material in an atmospheric phase. The substrate is positioned at a distance from the spray apparatus where the particulate impinges the substrate at a velocity effective to induce transformation of at least a portion of the polymorphic materials to a nanocrystalline, high pressure phase. If desired, the velocity can be greater than said velocity effective to induce transformation of at least a portion of said particulate to said nanocrystalline, high pressure phase. Moreover, the particulate can be dispersed in a carrier gas prior to being introduced into the high-velocity gas jet.
The particulate can have a particle size from 1 to 100 xcexcm, with 5 to 50 xcexcm being preferred. The particulate can be a semiconductor, a semiconductor precursor, a ceramic material, a ceramic/metal-based material, and combinations thereof. Semiconductors include, but are not limited to, silicon, germanium, doped derivatives thereof, and combinations thereof. Semiconductor precursors include, but are not limited to, graphite. Ceramics include, but are not limited to, silicon oxide, silicon nitride and silicon carbide.
The substrate to have the nanocomposite coating deposited therein is preferably an inorganic material, such as a metal or non-metal. One particularly preferred non-metal substrate is silicon. One particularly preferred metal substrate is steel.
The present also provides an article having a nanocomposite coating. The article is substrate having coated thereon the nancomposite coating which is a matrix of a polymorphic material in an atmospheric phase having dispersed therein nanocrystals of the polymorphic material in a high pressure phase. The coating can have a thickness from about 10 to about 500 micrometers. Preferably, the high pressure phase nanocrystals are at least 5 percent by volume of the nanocomposite coating, with at least 20 percent or at least 50 percent being more preferred. Preferably, the nanocrystals range in size from about 1 to about 100 nanometers, with about 5 to about 50 nanometers being more preferred.